1. Field of the Invention
The present invention relates to a supercharged pressure control apparatus to be used for an engine for automobiles equipped with supercharger.
2. Description of the Prior Art
The supercharger is a kind of air pump or blower installed in an inlet system of internal combustion engines to compress the intake air to a higher pressure than the atmospheric pressure in order to feed it into a cylinder. The object of installment of the supercharger is to increase the amount of air to be fed to the cylinder and to increase the output power that can be produced from an engine.
In an engine for automobiles with a supercharger, especially with a mechanically driven supercharger, it is necessary to optionally control the supercharged pressure according to operational conditions of the vehicle. In the conventional control system, there have been used a relief valve to control the supercharged pressure, as illustrated in FIG. 2, FIG. 3, FIG. 4 and FIG. 5. However, in the case of a supercharger for reciprocating engines such as shown in FIG. 2 and FIG. 3, the relief valve 2 operates to release air when the supercharged pressure exceeds a predetermined value, because the throttle 1 of the carburetor is closed at the time of reduced speed and idling, but the residual pressure remains in the surge tank 8 at time of reduced speed and idling and problems such as the occurrence of rough idling or the continuance of idling-up condition are caused, resulting in disadvantages such as an increase in fuel cost.
The supercharger in a reciprocating engine shown in FIG. 4 and FIG. 5 employs a system to control the relief pressure by utilizing the intake pressure of the intake manifold 4 to eliminate the aforementioned disadvantages. Since the pressure in the intake manifold 4 is negative at the time of reduced speed and idling, problems such as rough idling are overcome by lowering the operating pressure of the relief valve 2 by this negative pressure to lower the supercharged pressure in the surge tank 3.
In the case of the supercharger shown in FIG. 4 and FIG. 5, however, air is released when the pressure difference between the surge tank 3 and the intake manifold 4 has become greater than a predetermined value and at the time of reduced speed and idling, but when the engine 5 is running at a high speed (i.e. throttle in a fully open condition), the supercharged pressure in the surge tank 3 and the pressure in the intake manifold 4 become nearly the same, so that the pressure in the upper chamber A and lower chamber B of the relief valve 2 becomes the same, the relief valve does not open, and the surge tank 3 is charged with an abnormally high pressure, resulting in such disadvantages as knocking of the engine 5 and eventual damage of the engine.